RULES for recursive functions don't work properly

someone mentioned to me that this expression:

mapM_ action [n..m]

isn't being optimised properly, so I thought I'd look into it. Sure enough, we don't get down to a simple loop like we should, although the F/B transformation is happening (see ~simonmar/scratch/mapm.hs). The problem is that GHC.Enum.eftIntFB isn't being inlined, it's defined like this:

{-# INLINE [0] eftIntFB #-}
eftIntFB :: (Int -> r -> r) -> r -> Int# -> Int# -> r
eftIntFB c n x y | x ># y    = n	
		 | otherwise = go x
		 where
		   go x = I# x `c` if x ==# y then n else go (x +# 1#)

but, strangely, the inlining doesn't appear in the interface:

eftIntFB :: (GHC.Base.Int -> r -> r) -> r -> GHC.Prim.Int# -> GHC.Prim.Int# -> r
  {- Arity: 4 HasNoCafRefs Strictness: LLLL -}

This is becuase the RULE make eftIntFB look recursive, so its inlining isnt' exposed. Bad, bad.